Product Spectrum
The development of the product spectrum in time during typical operational cycles is shown in figure 3. In the 40-50 d SRT system the measured biomass and VFA produced covered for 90% the influent COD on average. For the VFA an average was made from day 71-136 as the product spectrum was constant in this period. In this period the product spectrum consisted mainly out of acetate and propionate produced in a ratio of propionate:acetate 2.05:1 (molpropionate:molacetate). At SRT-values of 1-2 d and 10-20 d the measured biomass yields together with the produced VFA covered for 92-94% the influent COD on average for the entire enrichment period. There were no significant amounts of unknown additional fermentation products formed as the measured soluble COD concentration corresponded to the sum of the measured compounds (0.97 ± 0.09 (n=55) gCODVFA·gCODSol-1). The increasing gap observed in the CODSol balance during glucose depletion, and subsequent increase of the CODSol balance due to secondary VFA production, suggests that part of the substrate was converted to a storage polymer. This storage polymer was not quantified, as no distinguishment could be made between storage polymer and extracellular polymeric susbtances (EPS) after extraction. When the SRT was shortened to 1-2 d, next to storage polymers, lactate was produced during the glucose consumption phase. Transiently produced lactate and storage polymers were converted to a mixture of acetate, propionate and butyrate. Inline gas measurements showed a CO2 partial pressure of 20-40% (m/m) at all SRT. H2 partial pressures measured ranged from 0.01% to 0.2 % and corresponded in all cases to less than 1 % of the COD supplied to the system.